Multilayer laminate heat sink assembly

ABSTRACT

A controlled oxygen content copper clad laminate product. In accordance with one aspect of the present invention, there is provided a laminate having a first layer of oxygen-free copper joined to a second layer of oxygen-rich copper by the steps of (i) cladding the first layer to the second layer at a relatively low speed to minimize rolling friction, (ii) finish rolling the laminate to substantially increase its thickness tolerance, (iii) slitting the laminate to increase its width tolerance, (iv) profiling a groove at a selected location in the laminate, (v) finish slitting a plurality of ribbons from the laminate, (vi) tension leveling the laminate to straighten and flatten its shape, (vii) stamping the laminate into sections each of a selected configuration, (viii) cleaning laminate surfaces, and (ix) direct bonding the laminate to a substrate material such that the first layer is annealed to the second.

This application is a continuation of application Ser. No. 08/182,288,filed Jan. 14, 1994, now abandoned.

FIELD OF THE INVENTION

The present invention relates to processes for joining materials to oneanother and more particularly to the production of a multilayer laminatefor energy transfer applications.

BACKGROUND OF THE INVENTION

Materials having relatively high thermal and electrical conductivitieshave been found desirable for use in microelectronic packaging. PureCopper, for instance, has been found particularly suitable both as anefficient conductor of electricity and because of its ability to rapidlyconduct and dissipate heat.

For this reason, copper sheeting is often used as part of a microchipheat sink/isolator. Typically, the sheeting is bonded to a ceramicsubstrate. This may be done by a process known as direct bonding. DirectBonding is notable for its products which have a relatively high thermaldissipation capability, strength, reliability, and small size. Thisprocess is also known for its relatively low cost. An intermediatebonding layer is sometimes used to modify the properties of the sheetingmaterial, e.g., to strengthen the material or control its coefficient ofthermal expansion.

Direct bonding requires that the bonding surface of the copper beoxidized so that covalent bonds can be formed with the ceramic duringthe thermal bonding cycle. Typically, both sides of the copper strip arecoated with (or dipped in) a chemical solution that promotes oxidation.This forms a low melting temperature eutectic of copper oxide on eachside of the strip which bonds to a ceramic substrate upon heating.

While one oxidized side of the strip is joined to the substrate duringdirect bonding, the other oxidized side bonds undesirably to the carrieror fixture upon which it rests. Also, the resulting bond has been foundof limited integrity, dimensional stability, durability and reliability.Chemical coating has additionally been found costly and producesexcessive chemical waste.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a controlled oxygen content copper clad laminate and process.The laminate comprises a first layer of oxygen-free copper joined to asecond layer of oxygen-rich copper by the steps of (i) cladding thefirst layer to the second layer at a relatively low speed to minimizerolling friction, (ii) finish rolling the laminate to substantiallyincrease its thickness tolerance, (iii) slitting the laminate toincrease its width tolerance, (iv) profiling a groove at a selectedlocation in the laminate, (v) finish slitting a plurality of ribbonsfrom the laminate, (vi) tension leveling the laminate to straighten andflatten its shape, (vii) stamping the laminate into sections each of aselected configuration, (viii) cleaning laminate surfaces, and (ix)direct bonding the laminate to a substrate material such that the firstlayer is annealed to the second.

Although the present invention is shown and described in connection withoxygen-free and oxygen-rich copper, it may be adapted for bonding othermaterials such as those containing precious metals, aluminum, titanium,nickel, steel, and their alloys as well as carbon and ceramics.

Accordingly, it is an object of the present invention to provide alaminate with enhanced heat transfer properties for use in highperformance heat sinks.

Another object of the present invention is to provide for the simple andefficient manufacture of heat sinks with minimum manual laborrequirements.

Still another object of the present invention is to maximize the energytransfer properties of materials comprising a multilayer laminate.

Yet another object of the present invention is to facilitate low costproduction of high performance heat sink structures having acceptableacoustics, cooling rates and pressure drop.

A further object of the present invention is to facilitate rapiddissipation of heat from microelectronic packaging.

Another object of the present invention is to isolate microelectronicmodules thermally and electrically from one another.

Still a further object of the present invention is to substantiallyreduce chemical waste during metal bonding.

The present invention will now be further described by reference to thefollowing drawings which are not intended to limit the accompanyingclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph of a controlled oxygen content copper cladlaminate at 120× magnification, in accordance with one aspect of thepresent invention;

FIG. 2 is a micrograph of the laminate of FIG. 1 direct bonded toberyllium oxide substrate;

FIG. 3 is a micrograph of oxygen-rich copper strip at 120× magnificationcontaining 380 ppm oxygen, in accordance with another aspect of thepresent invention;

FIG. 4 is a perspective view of a heat sink/isolator in accordance withone aspect of the present invention;

FIG. 5 is a plan view of a copper clad laminate strip. Shown in dashedlines is a fin stamped from the strip of FIG. 4;

FIG. 6 is a side view of the fin of FIG. 5;

FIG. 7 is a plan view of a copper clad laminate strip in accordance withanother aspect of the present invention;

FIG. 8 is a sectional view taken along lines 8--8 of FIG. 7;

FIG. 9 is a representation of the laminate and substrate of FIG. 8 priorto direct bonding;

FIG. 10 is a plan view of a piece cut from the strip of FIG. 7;

The same numerals are used throughout the figure drawings to designatesimilar elements. Still other objects and advantages of the presentinvention will become apparent from the following description of thepreferred embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Microelectronic packaging such as multi-chip modules (MCMs) have beenfound advantageous for providing relatively high power with reducedinterconnection signal delay and packaging volume, as compared toconventional single-chip modules. Given their substantial heat fluxes,stacked or laminated heat sinks with parallel channels have been foundnecessary to obtain heat sink dimensions suitable for effectivedissipation of heat.

Laminated heat sinks typically have a base with an array of stacked,plate-like fins. Each fin is configured such that a channel is formedbetween each fin pair. Heat is first conducted from the base to thefins, and then dissipated from the fins by thermal transfer to a coolantsuch as air passing through the channels. A heat sink of this generalconfiguration is shown in FIG. 4.

Increased device and module powers require high performance,fluid-cooled heat sinks. While increased heat dissipation from the finsis generally achieved by increasing the coolant flow rate, the flow ratenecessary for optimal cooling has to undesirable flow-inducedvibrations, pressure drop, and acoustics which may effect the sensitivemicroelectronics nearby.

To form a laminated heat sink, typically a plurality of copper plates,each having a wide shallow open channel across one face, are joinedfront-to-back. In the laminate, the channels become an array of closedparallel cooling channels. By adjusting plate thickness, as well as thewidth and depth of the open channel across the face of the plates, heatsink dimensions and, hence, heat fluxes are controlled.

In other environments, heat sinks are used to isolate microchips boththermally and electrically from one another. For example, a microchip issoldered to a heat sink in the form of a planar, multilayer laminatewhich, in turn, is mounted to a microelectronics module. An objective isto prevent differing levels of heat and electrical energy generated byeach microchip from interfering with the operation of other chips of themodule.

The foregoing description is provided for purposes of illustration andnot to limit the intended environment or application of the presentinvention. The remaining structural and functional aspects ofmicroelectronic packaging are known by those skilled in the art andfurther description is believed unnecessary for illustration of thepresent invention.

In accordance with one aspect of the present invention, there isprovided a controlled oxygen content copper clad laminate and process.The laminate comprises a first layer of oxygen-free copper joined to asecond layer of oxygen-rich copper by the steps of (i) cladding thefirst layer to the second layer at a relatively low speed to minimizerolling friction, (ii) finish rolling the laminate to substantiallyincrease its thickness tolerance, (iii) slitting the laminate toincrease its width tolerance, (iv) profiling a groove in the laminate ata selected location, (v) finish slitting a plurality of ribbons from thelaminate, (vi) tension leveling the laminate to straighten and flattenits shape, (vii) stamping the laminate into sections each of a selectedconfiguration, (viii) cleaning laminate surfaces, and (ix) directbonding the laminate to a substrate material such that the first layeris annealed to the second.

Initially, certified oxygen-free copper strip, e.g., OFHC copper 99.99%Cu (Mill Standard C-101), is processed in coil form from a selectedstarting size, e.g., of about 0.060 in.×2,500 in. (annealed temper). Thestrip is cold rolled using a calculated pass schedule, e.g., from about0.060 in. (spring temper UTS 50 KSI minimum) to 0.025 in. and from about2.500 in. to 2.130 in. Preferably, size reduction is limited to about20% for a single pass to prevent excessive temperatures, e.g., 200° F.,which could effect the grain structure of the copper. The resultinggrain structure has been found relatively important both functionallyand cosmetically to the grain size of the final laminate product.

An oxygen-rich copper strip, e.g., electrolytic tough pitch (ETP) copper99.90% Cu (Mill Standard C-110), is also processed from a selectedstarting size, e.g., of about 0.125 in.×2.500 in. (annealed temper). Thestrip is cold rolled, for example, from about 0.060 in. (extra springtemper UTS 58 KSI minimum) to 0.025 in. and from about 2.500 in. to2.130 in. The oxygen content of Mill Standard C-110 is generally withina range of 10 ppm to 2000 ppm. For the most consistent results in bonduniformity and strength, it is preferred that the oxygen content becontrolled to remain within a selected envelope, e.g., generally withina range of 300 ppm to 500 ppm oxygen. In one embodiment of the presentinvention, the concentration of oxygen is about 380 ppm, as shown inFIG. 3. Oxygen appears as heavy black spots at the grain boundaries.

Next, the surfaces of the oxygen-free copper strip and the oxygen-richcopper strip are cleaned for bonding. This is done by passing the stripsthrough an aqueous-ultrasonic degreasing machine to remove organicmaterials (e.g., oil and dirt) and then under a rotating soft abrasivewheel (brush) flushed continuously with water (wet brush). This removessurface oxides of the copper and roughens its surface to enhancecladability.

The oxygen-free copper strip is then placed atop and joined to the stripof oxygen-rich copper by cladding, e.g., mechanically crushing thestrips together in a 4-hi rolling mill. The resulting thicknessreduction and rolling friction typically produce heat. Again, sinceexcessive heat could affect grain structure, it is preferred that thestrip be simultaneously air quenched at the exit end of the rollingmill. In addition, relatively low process speeds, e.g., less than orequal to about 12 ft./min., are used to prevent heat build-up.Alternatively or concurrently therewith, the percent (%) reductionduring cold roll bonding is also limited, e.g., to about 76%.

The first layer preferably comprises copper sheeting having an oxygencontent generally within a range of 95 ppm to 2000 ppm. As the oxygencontent decreases, bond strength increases, a maximum strength being atabout 95 ppm. It is also preferred that the second layer compriseoxygen-free copper sheeting, i.e., copper containing a maximum of about10 ppm oxygen.

During the next step, the laminate is finish rolled to substantiallyincrease its thickness tolerance. Finish rolling is also done on a 4-hirolling mill. This step improves substantially the thickness toleranceof the laminate, e.g., from about ±0.001 in. to 0.0003 in. When highlypolished rolls are used, surface finish is also improved.

The laminate is then slit to increase its width tolerance. Duringslitting, the laminate is passed between two rotating arbors carryingrotating cutting blades. This removes the edges of the laminate, therebytrimming its width. Alternatively or concurrently therewith, apre-slitting step is performed prior to slitting. Pre-slitting removesrough edges and improves width tolerance of the laminate, e.g., fromabout 0.015 in. to 0.001 in. Improved width tolerance has been foundrelatively important for guiding the strip smoothly and precisely duringprofiling.

Upon the fourth step, at least one groove is profiled at a selectedlocation in the laminate to provide a flow channel. As coolant (liquidor gas) flows through the channel, heat removal is effected. Hence, inaccordance with one aspect of the present invention, the groove acts asa heat exchanger. Alternatively or concurrently therewith, the grooveserves as a reservoir for capturing excess solder during assembly by theend user.

Profiling may be done by form rolling, skiving, or milling. Duringskiving, for instance, the material is passed under a stationary cuttingblade known as a skiving tool. This changes the laminate thickness inselected areas across the strip width. Alternatively, the laminate isform rolled, i.e., passed under a roll with a continuous protuberancehaving the dimensions of the groove.

During the fifth step, a plurality of ribbons are finish slit from thelaminate. Finish slitting is done preferably using a technique like thatdescribed in step three, but to cut individual ribbons from a widerprocessing width and yield a multiple of strands.

Thereafter, the laminate is tension leveled to straighten and flattenits shape. Tension leveling is done, for example, by placing thelaminate in tension, that is, passing it through a series of deflectionrolls, e.g., nine, each of a relatively small diameter, e.g., about0.375 in. At this stage, the grain structures of both the layers ofoxygen-free copper and oxygen-rich copper are substantially unaffected,as shown in FIG. 1.

This step enhances strip quality by improving straightness, flatness,and shape of the strip with minimal variation. By insuring intimatecontact between the laminate and ceramic substrate, void-free bonds areformed. Void-free bonds are relatively important for effective directbonding.

Subsequently, the strip is stamped into discrete pieces, cleaned(degreased) to eliminate oils and other materials which may interferewith direct bonding, and bonded to a ceramic substrate by directbonding. Cleaning is done preferably by abrasive polishing and/orchemically etching the pieces. For instance, the stamped pieces areplaced in a vibratory bowl with abrasive ceramic media, e.g., XC3-8Tangle cut triangles. Next, the pieces are removed and chemically etched,e.g., using a series of acid baths--the first bath being a hydrocholoricor sulfuric acid solution, e.g., Branson MC-2, followed by a bath ofammonium persulfate, and a citric acid bath.

During direct bonding, temperature and other conditions simultaneouslyanneal the laminate, metallurgically bonding the first layer to thesecond. After direct bonding, a fine grain structure is produced in bothcopper layers (See FIG. 2). Various methods of direct bonding metals toceramics are described, for example, in U.S. Pat. No. 3,944,430, whichissued on Nov. 30, 1976, and in U.S. Pat. No. 4,129,243, which issued onDec. 12, 1978, the disclosures of which are hereby incorporated byreference.

Prior to direct bonding, layers were joined mechanically by cladding.This produces a "green bond" which has generally been found of marginalintegrity. Strengthening is typically done metallurgically by continuousstrip annealing prior to direct bonding. The present process isadvantageous in that annealing is done during rather than prior todirect bonding so that the effects of cold working are maintained, i.e.,it facilitates production of an ultra-fine grain size in the layersduring direct bonding. Because one side of the laminate remainsoxygen-free, no bonding occurs between the laminate and the setterplate, fixture or other carrier. The oxygen-free side can then sit onthe setter plate during subsequent firing of direct bond coppercomponents.

Also, oxygen impregnated copper provides oxygen to the copper-ceramicinterface more uniformly than does chemically coated copper. Thisimproves substantially bond integrity and part reliability. Moreover, iteliminates the three chemical baths required to chemically coat copper,significantly reducing costs, chemical waste, and the health risksassociated with hauling the waste. Dimensional stability and durabilityof oxygen impregnated copper is also superior.

Yet another advantage is the decreased grain growth accomplished by thepresent invention. This results in a substantially smoother, void-freesurface finish, as compared to chemically oxidized copper or straightETP. It also prevents build-up of eutectic oxide on the post-firedsurface, eliminating the need for chemical stripping of the coppersurface before solder application.

In accordance with one embodiment, the stamped pieces of laminate arestacked one on top of the other such that, during direct bonding(annealing), diffusion bonding occurs between the internal metal tometal interfaces of the clad layers. Simultaneously, there is diffusionbonding at the metal to metal interfaces between adjacent sides of thestacked laminate pieces. This substantially eliminates voids between theinterfaces. The result is a void-free, multiple-layer copper cladlaminate with maximum heat transfer capability.

Overall, this lamination technique advantageously permits cost-effectivemass production of high performance heat sink structures, significantlyextending cooling and isolation characteristics for microelectronicapplications.

Although the embodiments illustrated herein have been described for usewith copper or a copper alloy, it is understood that an analogousprocess could be practiced on other materials, giving consideration tothe purpose for which the present invention is intended. For example,similar processing of materials containing precious metals and theiralloys, aluminum, titanium, nickel, steel, carbon and ceramics isconsidered within the spirit scope of the present invention. It is alsoforeseeable that an intermediate bonding layer such as nickel, titaniumor silver could be added between the copper layers to modify propertiesof the laminate materials.

By the forgoing process, a laminate 10 (or preform) is produced whichcomprises a layer 11 of oxygen-free copper (C10100) clad to a layer 12of oxygen-rich copper (C11000), as shown in FIGS. 8 and 9. In accordancewith one aspect of the present invention, the laminate is a strip 13about 0.010 in. thick and about 0.500 in. wide. As shown in FIGS. 7-10,a channel 14 is profiled in the strip about 0.003 in. deep±0.0006 in.and about 0.036 in. wide which runs generally along the center of thestrip, about 0.210 in. from one side of the strip and about 0.174 in.from the other side.

During direct bonding, the laminate is bonded to a ceramic substrate 15such as beryllium oxide so as to produce a laminate product 18. In oneembodiment, a "dog bone"-like shape 16 is formed in the laminate such asby etching or stamping. Neck 17 of the "dog bone" extends about 0.0180in. from each side of the channel center line, and begins about 0.040in. from the strip center line.

This product has been found effective as a heat sink/isolator formicroelectronic modules. For instance, the % strength retention of thisBeO-DBCu metallized substrate after thermal cycling has been foundsubstantially superior to that of conventional Brush Wellman Mo-Mn orDUPONT 0022.

The present invention is also beneficial in permitting the use of purecopper which has a substantially higher thermal conductivity than thatof Cu thick film, Mo-Mn thick film or As-Pd thick film and asubstantially lower resistivity. The use of DBCu, it is noted, provideselectrical conductivity within about 5% of that of pure copper.

The BeO substrate has similar benefits in having a thermal conductivitysubstantially higher than that of AIN. Together, the copper laminate andBeO substrate provide superior thermal resistance performance overAIN-Cu or AI2O3-Cu. A substantially higher maximum conductor current isalso provided without the need for an intermediate bonding layer.

In accordance with another aspect of the present invention, each DBCuproduct is used as a fin 21 of a multilayer heat sink 20, as shown inFIG. 4. Each fin is stamped from multi-gauge DBCu strip having a channel22, e.g., about 103 mm wide, generally along its center such thatspacers 23 are left on opposing sides of the fin, as best seen in FIGS.5 and 6. Each fin has selected dimensions, e.g., about 115 mm wide andabout 1.5 mm thick.

The fins are stacked upon one another so as to form a block and joined(or fixtured) together by conventional thru rivets 24. The rivetspreferably comprise a material substantially the same as that of thefins. In the present embodiment, the rivets substantially compriseoxygen-rich copper (C110) and have selected diameters, e.g., eachgenerally within a range of 0.003 in. to 0.006 in. It is understood thatthe necessary dimensions and configuration of the rivets depends uponblock size and other structural requirements, as will be understood bythose skilled in the art.

At least one stiffener rib 25 is positioned along the center of channelfor added reinforcement. The stiffener rib divides the channel into twoequal portions, each about 50 mm wide.

During direct bonding, there is interdiffusion (diffusion bonding) notonly at metal to metal interfaces between the fins, but also between therivets and the fins. This results in the formation of a solidified heatsink structure.

By the present invention, the energy transfer capability of heat sinkstructures is maximized while maintaining acceptable acoustics, coolingrates and pressure drop for the sensitive microelectronics nearby, and alow cost.

Various modifications and alterations to the present invention may beappreciated based on a review of this disclosure. These changes andadditions are intended to be within the scope and spirit of thisinvention as defined by the following claims.

What is claimed is:
 1. A heat sink/isolator for microelectronicpackaging, which comprises:a plurality of fins stacked upon one another,each fin including a layer of a first material clad to a layer of asecond material and a groove at a selected location; a plurality ofrivets passing through the stacked fins, each rivet having a materialcomposition substantially the same as the second material; material tomaterial interfaces between adjacent stacked fins, clad layers of eachfin, and the rivets and fins being diffusion bonded to one another so asto form a solidified heat sink structure.